A twisted ring of cold gas and dust
spans over 600 light-years around
the galactic center
(more).

A twisted, elliptical ring of relatively dense gas and dust
around the
Milky Way's
center has been observed by the infrared
Herschel
Space Observatory. While a few portions of the ring, which
stretches across more than 600 light-years (semi-major axes of
100 adn 60 parsecs), were observed before,
Herschel's images in relatively long sub-millimeter, far-infrared
wavelengths reveal the entire ring for the first time. Containing
around 30 million Solar-masses of matter, the ring lies in the
Central
Molecular Zone within the Central
Bulge, a region rich with cold molecular clouds of gas
and dust moving within elongated orbits around the
Galactic
Bar.
Oriented perpendicular to the major axes of the Galactic
Bar,
the ring contains gas and dust at a temperature of just 15
degrees above absolute zero, and bright regions depict denser
masses of gas and dust that include some of the most massive
and active sites of star formation in the galaxy. Past
computer modelling have shown that galactic bars and rings
can be formed by gravitational interactions, and such
structures in the Milky Way's core even may be related to
interactions with our largest neighbor, the
Andromeda Galaxy. (Herschel
news
release; and
Molinari et al, 2011).

A ribbon has been superimposed
over the twisted ring of cold
gas and dust around the galactic
center
(more).

The central region of the Milky Way is dominated by a
bar-like structure, which stirs up the material in the
outer galactic disk as it rotates over millions of years
and may be responsible for its spiral structure.
Galactic
bars are density waves that rotate around spiral
galaxies with a speed different from the rotation
speed of individual stars in the dense central bulge,
much like the waves that create the spiral structure of
these galaxies. These density waves may be created by
gravitational instabilities in the centers of galaxies,
or by gravitational disturbances from nearby galaxies.
As the waves rotate around a galaxy, they hold their
shape like the blades of a fan. Gas closer to the
galatic center moves faster gas farther out, which,
combined with density waves, helps create the bar.

This 60 to 70 ly-wide image depicts a region
of the galactic core that includes a supermassive
black hole associated with a compact radio and
x-ray source called Sagittarius A* (Sgr A*),
located by the flare at center which took only
minutes to brighten before fading after three
hours in October 26-27, 2001 (more at
Chandra
X-Ray Observatory and
Baganoff
et al, 2001).

The galactic core is anchored by a black hole that has been
most recently calculated to have around 4.31 million (+/- 0.06)
Solar-masses
(Gillessen et al,
2008). Based on theoretical
calculations, its event horizon is estimated to be about
15.4 million km (16.5 million miles or 52 light-seconds)
wide -- about 11 times Sol's
diameter. Despite its size, the hole appears to be spinning
so fast that it completes each revolution in about 11 minutes,
at around 30 percent the speed of light. Although the hole
is associated with the compact, radio, and x-ray source
Sagittarius
A* (Sgr A*), its faintness at all wavelengths, even
in x-rays, has been puzzling as astronomers expected it
to shine more brightly from infalling matter. The hole
does occasionally
flare
from apparent infalling of large chunks of matter,
but it appears that the Milky Way Galaxy's central black
hole is relatively quiescent at present. Analysis of
radio emissions from the area around the hole suggests
that it is currently "starved" of matter -- as there is
little "twisting" of radio waves from passing through
magnetized plasma -- perhaps because a recent supernova
cleared out matter around the black hole (more from
Astronomy
magazine).

Cloud of hot gas in the innermost
10 light years of the galactic core
of the Milky Way is heated by
supernova shockwaves and possibly
colliding winds from young massive
stars
(more
information).

In 2003, two astronomers (Brad
Hansen and Milos Milosavljevic) announced the probable existence of a
middleweight black hole with thousands of Solar-masses that is dragging a
cluster of young stars towards the Milky Way's supermassive, central black
hole. Although the central black hole should be too massive to allow any
new stars to form within three or four light-years because its gravity would
tear apart any large clouds of gas and dust that would otherwise condense
into stars, a cluster of stars less than 10 million years old has been
observed to be located less than half a light-year from the central black
hole. The cluster probably formed at a safe distance of at least five
light-years away from the black hole, around a middle-weight black hole with
around 1,000 to 10,000 Solar-masses. As the cluster's black hole has been
gravitationally dragged towards its supermassive neighbor, however, it has
dragged its stars along with it.

The cluster has been orbiting the supermassive central black hole
roughly once every 100 years. As the cluster spirals in towards the
central black hole, however, it has been flinging out some of its stars
to shed angular momentum as it spirals in. Eventually, it will fall in
and add its mass into the larger central black hole. These infalls of
mass should have occurred many, many times during the development of the
Milky Way
(Hansen and
Milosavljevic, 2003).

In 2004, a team of astronomers
(Jean-Pierre Maillard,
Thibaut
Paumard,
Susan R. Stolovy, and
François Rigaut)
announced the possible discovery of a middleweight black hole with around
1,300 Solar-masses orbit about three light-years from the central black
hole, called GCIRS 13E, with strong x-ray emission
(press
release);
nature.com;
and Maillard et al, 2004).
A compact cluster of seven massive stars (spectral type O and Wolf-Rayet,
as well as a luminous blue object), each with five to 10 Solar-masses but
reduced from more than 40 Solar-masses during their prime, orbit GCIRS 13E.
Just 0.065 light-years (0.02 parsecs) across, GCIRS 13E is racing around
the galactic center at 626,300 miles per hour (280 kilometers per second).
Indeed, the gravity of the nearby, supermassive central black hole should
have precluded the contraction of gas clouds into GCIRS 13E's stars. The
astronomers believe, however, that the stars did form too far from their
present location because the seven massive stars must be less than 10
million years old, or they would have exploded already since such massive
stars do not live that long. Hence, the seven stars and GCIRS 13E must
have migrated inward toward the central black hole within the past 10
million years, probably from about 60 light-years further out its current
orbit. The seven stars are the remains of what likely was once a much
larger and massive cluster of many stars, possibly a globular cluster
where a middleweight black hole could develop through runaway star
collisions, as indicated by other research.

The Milky Way's spinning black hole drags space around
with it, at right,
allowing infalling matter to orbit closer than a
non-spinning black hole, at left.

The gravity of a dense disk of gas (of at least 10,000
Solar-masses) around the central hole in Sagittarius A*
apparently counters its tidal forces sufficiently for
stars to form. Moreover, local conditions have also
favored the formation of a high number (around 100) of
very young and massive stars. A corresponding failure
to detect around a million of smaller Sol-sized stars
suggest that the large stars formed locally, rather
than farther away from the central black hole before
inward migration to the region of the gas disk
(more).

Large stars of 30 to 50 Solar-masses have apparently formed in a dense
disk of gas within 0.3 light-years of the central black hole
(more).

The Star

S2 is apparently a "normal" star
(ESO
press release).
It has about 15 times Sol's mass and seven times its diameter,
and so, if it is a main sequence star, then S2 may be of
spectral and luminosity type (early) B V (or even
type
O V).
Given the frequency of supernovae at the galactic core, it is
likely to be relatively more enriched than Sol with elements
heavier than hydrogen ("metallicity"), based on its abundance
of iron.

This two light-year square image
of the innermost region of the
galactic core depicts hot stars
in blue and cool stars in red,
and two arrows pointing towards
Sgr A* and S2
(more at ESO
and Astronomy
Picture of the Day).

The star orbits the galaxy's central black hole at an average
distance of 5.5 light-days that takes about 15.2 years to complete,
at "an inclination of 46 degrees with respect to the plane of the sky"
(MPE
research introduction).
Its highly elliptical orbit is fairly stable, as S2 would have to
come 70 times closer to the hole (16 light-minutes) to be at risk
of being destroyed by tidal forces from the hole's gravity. Early
in 2002, S2 came very close to the black hole, coming within 17
light-hours or around three times the orbital distance of Pluto
from the sun (or 39 AUs). At its nearest approach, it zooms
around the black hole at speeds exceeding 11 million mph (5,000
km per second). Because of its extremely high eccentricity
(e= 0.87) of orbit, however, S2 also moves as far away from the
hole as 10 light-days. (See an infrared, time-lapse movie of the
very fast motions of stars near the central black hole at
Astronomy
Picture of the Day.)

The extremely elongated orbit
of S2 around the black hole
takes about 15.2 years to
complete
(more
information).

It is likely that any protoplanetary bodies that may have formed around
S2 are still agglomerating other planetesimals, or have been lost to
interstellar space or the black hole from gravitational perturbations.
In any case, any developing carbon-based life on a developing Earth-size
planet would be subject to tremendous heat and intense asteroidal and
cometary bombardment, in addition to the intense and deadly radiation
produced by supermassive black hole and nearby supernovae and other
massive young stars.

Other
Information

Neil Price has assembled images and maps of the
galactic region around
Sagittarius
A*. In addition, images and a brief movie of a
powerful infrared flare from hot gas falling into the
central black hole on May 9, 2003 are
available
from ESO.

Sagittarius is Latin for "archer," often represented as a centaur
wielding a bow and arrow since ancient times. The constellation
also contains the
Great
Sagittarius Star Cloud, where a
vast milky swarm of millions of stars mark the way to the center of
the galaxy. For more information about the stars and objects in
this constellation and an illustration, go to Christine Kronberg's
Sagittarius.
For another illustration, see David Haworth's
Sagittarius.

For more information about stars including spectral and luminosity
class codes, go to ChView's webpage on
The Stars of
the Milky Way.